1. Field of the Invention
The present invention relates to an uninterruptible power supply (UPS), and more particularly to a UPS having efficient power conversion.
2. Description of the Related Art
With reference to FIG. 7, a conventional UPS has an AC to DC conversion and charging circuit 60, a first DC conversion circuit 61, a second DC conversion circuit 62, a battery circuit 63 and a control circuit 64.
The AC to DC conversion and charging circuit 60 has a set of input terminals and a set of output terminals, and the set of input terminals is connected to an AC mains 65 to lower voltage of the AC power and convert the AC power into a DC power. The AC to DC conversion and charging circuit 60 has a low frequency transformer to lower voltage of the AC power.
The first DC conversion circuit 61 has a set of input terminals and a set of output terminals, and the set of input terminals is connected to the set of output terminals of the AC to DC conversion and charging circuit 60 to receive the converted DC power and lower the DC power with a voltage level, such as tens of volts, to that with another voltage level, such as 12 volts, so as to supply power to a first load 66.
The second DC conversion circuit 62 has a set of input terminals and a set of output terminals, and the set of input terminals is connected to the set of output terminals of the first DC conversion circuit 61 to further lower power outputted from the first DC conversion circuit 61 to that with different voltage level, such as a voltage drop from 12 volts to 5 volts, so as to supply power to a second load 67 with different power requirement. Hence, the first DC conversion circuit 61 and the second DC conversion circuit 62 output power with different voltage levels and supply the power to loads with different power requirements.
The battery circuit 63 is parallelly connected between the AC to DC conversion and charging circuit 60 and the first DC conversion circuit 61, and has a switching element 631, a diode 630 and a rechargeable battery 632. The switching element 631 has a control terminal and may be a relay. The diode is parallelly connected with the switching element 631. The rechargeable battery 632 is serially connected to the parallelly connected diode 630 and switching element 631. The cathode of the diode 630 is connected to one terminal of the set of output terminals of the AC to DC conversion and charging circuit 60, and the anode of the diode 630 is connected to the anode of the rechargeable battery 632. The cathode of the rechargeable battery 632 is connected to the other terminal of the set of output terminals of the AC to DC conversion and charging circuit 60.
The control circuit 64 is connected to the AC to DC conversion and charging circuit 60 and the control terminal of the switching element 631.
Given the foregoing circuit design, despite a power failure of the AC mains 65, the rechargeable battery 632 can still supply power to the first and second loads 66, 67 through the diode 630, thereby preventing the first and second loads 66, 67 from losing power.
To avoid a low power storage capacity of the rechargeable battery 632 failing to supply sufficient power at power outage, the control circuit 64 can monitor the power storage capacity of the rechargeable battery 632. If the power storage capacity of the rechargeable battery is detected to be lower than a preset value, it indicates that the power storage capacity of the rechargeable battery is low. The control circuit 64 then controls the switching element to be closed so that the DC power outputted by the AC to DC conversion and charging circuit 60 is not only outputted to the first DC conversion circuit 61 but also charges the rechargeable battery 632. If the power storage capacity of the rechargeable battery 632 is higher than the preset value, it indicates that the power storage capacity of the rechargeable battery 632 is almost saturated. The control circuit 64 then controls the switching element to be opened so as to stop charging the rechargeable battery 632. Hence, it is the control circuit 64 monitoring and controlling the rechargeable battery 632 on a real-time basis to ensure that the power storage capacity of the rechargeable battery is sufficient.
To sum up, the DC power received by the first and second loads 66, 67 is obtained by converting the mains power at least twice. In other words, the DC power received by the first and second loads 66, 67 at least passes through the AC to DC conversion and charging circuit 60 and the first DC conversion circuit 61. When the rechargeable battery 632 has a sufficient power storage capacity and is not required to be charged, the control circuit 64 controls the switching element to be opened so as to prevent the AC to DC conversion and charging circuit 60 from charging the rechargeable battery 632. As the input power of the first DC conversion circuit is still supplied by the AC to DC conversion and charging circuit 60, the additional stage of voltage conversion leads to worse power conversion efficiency. Moreover, a low-frequency transformer in the AC to DC conversion and charging circuit 60 further results in excitation loss. Under the foregoing circuit design, the AC to DC conversion and charging circuit 60 constantly performs DC voltage conversion and thus gives rise to more energy loss.